The electric powered passenger vehicle has long been considered one of the most attractive alternatives to conventional internal combustion engine vehicles from the standpoint of overall efficiency, environmental impact and, most recently, alternative fuel capability. Many commercial enterprises and private individuals, some under the auspices of the Federal Government, have proposed various approaches to implementing an electrically powered vehicle. To date, there have been virtually no commercially successful vehicles produced on a large scale. A large number of approaches to the implementation and control of an electric vehicle are evidenced in the patent literature. Most of the approaches fall within one of three general categories of motive power source. These categories are hybrids, DC motor drives and induction motor drives. The first type, that is most frequently found in the patent literature, is the hybrid vehicle, comprising a small gasoline fueled internal combustion engine which mechanically drives an electrical generator which, in turn, supplies electrical energy to an AC or DC motor. With this arrangement, the gasoline engine can operate at a constant speed (at a relatively high efficiency) and achieve a substantial fuel saving compared with an engine experiencing the conventional wide range of operation. A shortcoming of many hybrids is that they are relatively heavy, requiring an electrical generator and motor as well as the gasoline engine. Additionally, the engine requires substantial amounts of volatile liquid fuel and generates exhaust emissions.
A second approach taken in the development of electric vehicles is the use of a bank of batteries which supply electrical energy to a DC motor. A variable speed motor drive circuit provides easy and versatile control of a vehicle. The principle advantage of this arrangement is that a DC motor control system requires a relatively simple power and control circuit. Unfortunately, this advantage is often more than offset by the relatively large initial cost and maintenance expenses of the motor itself. In addition, DC machinery is relatively heavy and bulky, factors which do not lend themselves well to implementation within a lightweight compact vehicle. Finally, DC motors inherently require choppers and commutators which create sparks and RF pollution which can be controlled only at additional expense.
The third, and most attractive approach from the applicant's viewpoint, is a vehicle employing a battery bank and an AC motor. AC motors are relatively lightweight, inexpensive and efficient when compared to DC motors. AC motors, with no brushes or commutators, are more rugged and reliable than their DC counterparts and require substantially less maintenance. Related to the power-to-weight ratio is the fact that AC machines can be driven at substantially greater speeds than DC motors. Because AC motors do not generate sparks, they can readily be employed in dusty, explosive and highly humid atmospheres or high altitudes. Additionally, AC motors can be liquid cooled if the application so requires. Although typically superior to DC motors in electric vehicle applications, AC motors often require complex control circuits which are dedicated to associated vehicle drivetrains and can be extremely bulky and expensive. To date, virtually all AC electric vehicles have employed multi (usually three) phase design strategies. Although three-phase machinery has many advantages as set forth hereinabove, three-phase inverter costs and complexity have proven to be extremely high. In relatively large load applications, such as that required in a passenger vehicle, appropriately sized solid state switching devices such as SCR's or transistors are often extremely expensive. In addition, three-phase inverters, by their nature, dictate a multiplicity of components, including switching devices, again increasing system cost.
Multi phase machines of conventional design, although having inherent advantages over DC motors, have shortcomings of their own, particularly in traction drive applications. The geometry of many multi phase machines necessitate the use of expensive alnico or rare earth permanent magnets as well as elaborate mounting hardware. Additionally, winding the armature can be an expensive and difficult process. Because multiple discrete sets of coils are required in multi phase machines, the limited winding area available can result in excessive copper losses in the machine. Finally, the armature pole pieces of multi phase machines may operate at less than their full flux carrying capability if inexpensive low induction magnets are used internally, resulting in unnecessary weight and size.
It will be apparent from the reading of the specification that the present invention may be advantageously utilized in many different traction drive applications, especially land vehicles. However, the invention is especially useful when applied to electric powered passenger commuter vehicles, and will be described in connection therewith.
One of the difficulties encountered with prior electric motors is the relatively low motor torque available if the accelerator is pressed when the motor is below a minimum speed. The low motor torque is felt by the driver as a hesitation or lack of pick-up. While this condition is not dangerous, it is desirable to provide the electric vehicle with good pick-up from an at rest position, especially in vehicles designed to compete with gas and diesel powered vehicles.